Aphasia is impairment in a patient's ability to use language and may affect comprehension of speech, production of speech and the capacity to read or write. It occurs as a result of injury to the brain, commonly due to stroke, but also secondary to trauma, brain tumors, infection and dementia. Current estimates provide that 700,000 people suffer a stroke each year in the U.S. and that approximately two-thirds of these individuals survive and require rehabilitation. At least one-fourth of all stroke survivors experience language impairments involving the ability to speak, write, and understand spoken and written language. Aphasia, typically as a consequence of stroke, is estimated to affect 1 million Americans. (Albert, M L. Archives of Neurology 55(11)(1998) 1417-9.).
Aphasia has been categorized into several distinct diseases, including Broca's, Wernicke's, conductive, anomic and transcortical aphasia. In Broca's aphasia, also termed expressive, motor or anterior aphasia, comprehension is largely intact but oral and written communication is impaired. The patient thus communicates by nonfluent or impaired expression of either spoken or written language. Global aphasia is characterized by loss of all ability to communicate and typically results from extensive anterior-posterior lesions of the dominant (typically left) hemisphere.
Wemicke aphasia, also called receptive, sensory or posterior aphasia, is caused most often by occlusion of the lower division of the middle cerebral artery (MCA) bifurcation or one of its branches. These patients vocalize smoothly and with expression, but their speech is characterized by distorted phonetic structure, word substitution, and additional prefixes and suffixes. Although fluent in speech, the words produced are not understandable. Wernicke's area of the brain is located in the temporal lobe of the cortex near brain regions involved in processing sound inputs. The infarct responsible for a classic Wernicke's aphasia includes the dominant (typically left-sided) posterior temporal, inferior parietal and lateral temporal-occipital regions.
Conductive aphasia, also called associative aphasia, has been classically thought to be caused-by a disruption of the dominant (typically left) arcuate fasciculus or supramarginal gyrus extending to the temporal cortex. Patients with conductive aphasia have significant difficulty repeating unfamiliar phrases and words but demonstrate much better auditory and written comprehension compared to individuals with Wernicke's aphasia and are more likely to recognize the deficit and make an effort to self-correct. Damage to the language areas of the left hemisphere that are outside the primary language areas results in transcortical aphasia.
Anomic aphasia, also called nominal aphasia, also primarily influences the ability to fmd the right name for a person or object. Anomia is caused by damage to various parts of the parietal or temporal lobe and usually involves a breakdown in one or more pathways or connectivity patterns (diaschisis) between regions in the brain. Averbia is a specific type of anomia characterized by trouble remembering only verbs. Averbia is caused by damage to the frontal cortex in or near Broca's area.
In transcortical motor aphasia, transcortical sensory aphasia, and mixed transcortical aphasia, the patient has either partial or total loss of the ability to communicate verbally or using written words but is able to repeat words, phrases, or sentences.
Current accepted treatments for rehabilitation of aphasia include cognitive neurorehabilitation, computer-aided techniques, psycholinguistic theory-driven therapy, psychosocial management and pharmacotherapy. No pharmacotherapies are able to cure aphasia. Surgery is not useful in aphasia secondary to stroke, though aphasia due to a tumor or hematoma compressing a critical speech center may be responsive to surgery. Speech therapy is offered to aphasic patients for the purpose of developing full utilization of their remaining skills and to develop compensatory mechanisms for communicating.
Experimental therapies that have been proposed or tried in treatment of aphasia include transcranial magnetic stimulation (TMS). See, e.g., Epstein and Davey, U.S. Pat. No. 6,132,361. Experimental application of this technique for stimulation of region of the cortex shown to be hyperactive during aphasia has indicated significant improvement in naming pictures following 10 TMS treatments in a total of 4 patients. Martin P I, et al. Semin Speech Lang. 2004 25(2): 181-91. Similarly, TMS has been used to stimulate Wernicke's area, the right-hemisphere homologue of Wernicke's area, Broca's area, and the primary visual cortex in treatment of aphasia. Picture naming was reported to be facilitated after repetitive TMS. Mottaghy F M, et al. Neurology 53(8)(1999) 1806-12. Transcutaneous nerve stimulation by generation of trapezoidal mono-phasic pulses to the skin such as the skin of the arm was proposed in Butler et al., U.S. Pat. No. 4,431,000, but does not appear to have been sufficiently successful to have entered the treatment armamentarium for aphasia.
Further treatments for aphasia are clearly needed.